Multifrequency signaling is now commonplace in communications systems. One type of multifrequency signaling employs signals generated in accordance with a multifrequency code. The coded signal includes selected combinations of coincident two-tone burst, and each combination includes one tone from a relatively high-frequency band and one tone from a relatively low-frequency band.
Multifrequency signaling is employed in subscriber signaling, signaling between central offices, interoffice communication, remote control of other systems and entering data into computer systems. It is increasingly important that the generation of valid multifrequency signals be achieved accurately.
One prior method of testing the validity of multifrequency dialing signals involves splitting the multifrequency signal into high and low frequency groups using bandpass filters. Each of the frequency groups are amplified and clipped to form a square wave signal having the same period and the respective frequency group. Each of the square wave signals are then fed through divide-by-five counters. The outputs of these counters are used to gate two additional counters which are clocked by a reference clock. The additional counters are enabled by threshold detectors which determine if the amplitudes of respective frequency group signals exceed predetermined threshold values. Thus, when the threshold values are exceeded, the additional counters are enabled and count during intervals of five counts of each counter. The outputs of these counters are then hardwired through a series of logic gates to determine the characters being received.
In this technique, all of the frequency limits are predetermined and hardwired which are not subject to be easily changed. All amplitude limits must be manually set for the upper and lower limits which require switching circuitry for each amplitude limit. Moreover, there is no provision for self-calibration and the system required to implement the technique is very large and expensive.
U.S. Pat. No. 4,028,501, which issued to D. E. Rittenhouse, discloses a method and an apparatus for translating multifrequency signaling. Multifrequency signals are received and filtered into high and low bands. The signals in the high and low bands are used to derive high band and low band threshold levels, respectively. The signals in each band having amplitudes exceeding the threshold level in the band are limited and the signals of lesser amplitudes are inhibited. In each band, the frequency of the limited signals is digitally detected. An output signal, indicative of the frequencies detected, is generated solely in response to a substantially simultaneous and constant frequency detection in both bands extending over a preselected period of time. Signaling information is thereby translated from tone signals co-existing in any one of a plurality of predetermined frequency combinations for at least the preselected period of time and having substantially similar amplitudes.
In view of the foregoing, there is a need for a technique which accurately and economically tests the validity of multifrequency signals and which allows for self-calibration.